U.S. patent number 3,944,830 [Application Number 05/469,009] was granted by the patent office on 1976-03-16 for method of and an apparatus for examining a sample or a material by measuring the absorption of .gamma.- or x-ray radiation.
This patent grant is currently assigned to Aktiebolaget Atomenergi. Invention is credited to Erik Dissing.
United States Patent |
3,944,830 |
Dissing |
March 16, 1976 |
Method of and an apparatus for examining a sample or a material by
measuring the absorption of .gamma.- or x-ray radiation
Abstract
A method of and an apparatus for human bone density
determination in vivo. .gamma.- or x-ray radiation of two different
photon energies is scanned across the human body part to be
examined. For the two photon energies, the logarithms of the
intensities of the radiation transmitted through the human body
part are determined simultaneously. The logarithm values obtained
are processed on line such that the ratio of these values is
changed by a predetermined factor. The processed values are
subtracted from each other to produce a simultaneous output value
which is integrated to give an examination result proportional to
the bone tissue content expressed as weight per length unit in a
direction perpendicular to the scanning direction while being
independent of the surrounding soft tissue.
Inventors: |
Dissing; Erik (Nykoping,
SW) |
Assignee: |
Aktiebolaget Atomenergi
(Stockholm, SW)
|
Family
ID: |
20317447 |
Appl.
No.: |
05/469,009 |
Filed: |
May 10, 1974 |
Foreign Application Priority Data
|
|
|
|
|
May 11, 1973 [SW] |
|
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7306721 |
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Current U.S.
Class: |
378/55; 850/8;
378/56; 378/53 |
Current CPC
Class: |
A61B
6/482 (20130101); A61B 6/505 (20130101); G01N
23/083 (20130101) |
Current International
Class: |
A61B
6/00 (20060101); G01N 23/02 (20060101); G01N
23/08 (20060101); G01N 023/02 () |
Field of
Search: |
;250/308,358,359,360 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Archie R.
Attorney, Agent or Firm: Toren, McGeady and Stanger
Claims
What I claim is:
1. A method of examining a sample of biological tissue, which
sample includes at least two different substances, by measuring the
absorption of electromagnetic radiation, the method comprising the
step of: transmitting electromagnetic radiation including photons
of at least two different energies through the sample;
simultaneously determining, for each of two predetermined photon
energies, the logarithm of the radiation intensity of the radiation
passed through the sample; processing the logarithmic values
obtained so that the ratio of said values is changed by a
predetermined factor substantially equal to the ratio between the
mass absorption coefficient of one of said substances at the second
photon energy and the mass absorption coefficient of said one
substance at the first photon energy; and subtracting the
logarithmic values thus processed from each other in order to
obtain a simultaneous value so that the influence of said one
substance on the magnitude of the examination value is
eliminated.
2. A method according to claim 1, wherein the sample is scanned by
sweeping the radiation across the sample in a first direction while
continuously and simultaneously producing said examination
value.
3. A method according to claim 2, further including integrating,
during the scanning, the portion of the examination value produced
which is a function of the influence of said other substance on the
radiation, and obtaining a value which is proportional to the
content of the sample of said other substance in terms of weight
per length unit in a direction perpendicular to said first
direction.
4. A method according to claim 2, further including the step of
visually recording said examination value.
5. An apparatus for examining a sample of biological tissue, which
includes at least two different substances, comprising: a radiation
source emitting electromagnetic radiation having photons of at
least two different energies; detector means for determining the
intensity of radiation transmitted from said radiation source and
falling on the detector means, said detector means being arranged
to simultaneously produce a first signal having a value
proportional to the logarithm of the radiation intensity at a first
photon energy and a second signal having a value proportional to
the logarithm of the radiation intensity at a second photon energy;
means for processing said first and second signal so that the ratio
between said quantities is changed by a predetermined factor; and
means to produce an output signal having a value proportional to
the difference between said changed quantities of the processed
signals, said processing means being arranged to change said ratio
so that said value of the output signal is substantially
independent of the radiation influence of one of said substances
.
6. An apparatus according to claim 5, wherein said radiation source
and said detector means are arranged such that the radiation can be
swept across the sample while continuously producing the output
signal.
7. An apparatus according to claim 6, further including integrating
means for integrating a value proportional to said value of the
output signal.
8. An apparatus according to claim 5, wherein said detector means
include a photon detector producing a pulse for each incident
photon, the amplitude of said pulse being dependent on the energy
of the photon; a first discriminator arranged to pass pulses
corresponding to photons of said first energy level to a first
logarithmic ratemeter; and a second discriminator arranged to pass
pulses corresponding to photons of said second energy level to a
second logarithmic ratemeter.
9. An apparatus according to claim 8, wherein said logarithmic
ratemeters are arranged to produce a DC voltage, the amplitude of
which is proportional to the logarithm of the pulse rate.
10. An apparatus according to claim 9, wherein each of said
ratemeters is connected to a DC amplifier, the amplification
factors of said DC amplifiers differing from each other by said
predetermined factor, and wherein said DC amplifiers are connected
to a summing means such that the amplified DC voltages of the
ratemeters are subtracted from each other.
11. An apparatus according to claim 10, wherein each ratemeter is
connected to its amplifier via a circuit for adding a reference
signal such that the input voltage to the amplifier is zero when
the radiation from the radiation source falls directly on the
detector without passing through the sample.
12. An apparatus according to claim 10, wherein said summing means
are connected to a counter via a voltage-frequency converter.
13. An apparatus according to claim 6, further including recording
means for recording the magnitude of said value of the output
signal.
14. An apparatus according to claim 5, wherein said radiation
source includes a mixture of two isotopes.
15. An apparatus according to claim 14, wherein said two isotopes
are I 125 and Am 241.
16. An apparatus according to claim 5, wherein said radiation
source includes an isotope emitting photons of at least two
different energies.
17. An apparatus according to claim 16, wherein said isotope is Xe
133.
18. An apparatus according to claim 5, wherein said radiation
source is an x-ray tube, said detector means including window
discriminators adapted to function at said first and second photon
energies.
Description
The present invention relates to the examination of a sample or a
material, preferably the examination of the structure of biological
tissue, which includes at least two different substances, by
measuring the absorption of .gamma.- or x-ray radiation transmitted
through the sample. The invention is particularly applicable to the
determination in vivo of the mineral content in the human body
skeleton.
It is known to use in bone density determination in vivo a method
based upon the use of a radioactive radiation source emitting
photons of one specific energy. When using this method, the
so-called single isotope method, the body part to be exposed to
radiation is submerged in a liquid, for instance water, having
substantially the same linear absorption coefficient as has the
soft tissue surrounding the bone, the mineral content of which is
to be determined. In this way it is possible to a certain extent to
compensate for the radiation absorption in said soft tissue.
However, submerging the body part in a liquid means disadvantages
which reduce the applicability of the method.
Therefore, the so-called double isotope method has been developed,
which means that the test results of absorption measurements using
two different isotopes of different photon energies are combined
and calculated. In this way the influence of soft tissue can be
eliminated without any use of liquid surrounding the body part
examined. The specific content of the two substances is obtained by
calculating the separate test results of the two isotopes. The
calculation, generally being comparatively extensive, is suitably
carried out by means of a desk calculator.
Consequently, the object of the present invention is to provide a
method of and an apparatus for examination of the type of sample
mentioned above while using radiation of at least two different
photon energies, whereby the influence of one of the substances of
the sample on the examination result is simultaneously and
automatically eliminated, such that the examination result directly
gives the content of the other substance or substances.
The above-mentioned object is achieved in that the method and the
apparatus according to the invention show the features defined in
the appended claims.
Thus, the method according to the invention comprises transmitting
radiation including photons of at least two different energies
through the sample; simultaneously for a first and a second of two
predetermined photon energies determining the logarithm of the
radiation intensity of the radiation passed through the sample;
processing the logarithmic value obtained such that the ratio of
said values is changed by a predetermined factor; and subtracting
the logarithmic values thus processed from each other in order to
obtain a simultaneous examination value, said predetermined factor
being chosen such that the influence of one of the substances on
the magnitude of the examination value is eliminated. The ratio
between the logarithm value at the first photon energy and the
logarith value at the second photon energy is changed with a factor
being substantially equal to the ratio between the mass absorption
coefficient of said one substance at the second photon energy and
the mass absorption coefficient of said one substance at the first
photon energy. Suitably, the sample is examined by scanning the
radiation transmitted through the sample in a first direction while
continuously and simultaneously producing the examination value.
Advantageously, the examination value is simultaneously recorded to
produce a curve showing for instance a bone density profile. It is
also advantageous during the scanning to integrate the portion of
the examination value produced which is a function of the influence
of said other substance or substances on the radiation, whereby a
value is obtained that is proportional to the content of the sample
of said other substance or substances in terms of weight per length
unit in a direction perpendicular to said first direction.
The apparatus according to the invention comprises a radioactive
radiation source emitting .gamma.- or x-ray radiation having
photons of at least two different energies and detector means for
determining the intensity of radiation transmitted from said
radiation source and falling on the detector means and is
essentially characterized in that said detector means are arranged
to produce simultaneously a first signal having a quantity
proportional to the logarithm of the radiation intensity at a first
photon energy and a second signal having a quantity proportional to
the logarithm of the radiation intensity at a second photon energy,
in that means are arranged for processing said first and second
signal such that the ratio between said quantities is changed by a
certain factor, and in that means are arranged to produce an output
signal having a quantity proportional to the difference between
said changed quantities of the processed signals, said
first-mentioned means being arranged to change said ratio such that
said quantity of the output signal is substantially independent of
the influence of said one substance on the radiation. The radiation
source and the detector means are suitably arranged such that the
radiation beam can be scanned across the sample while
simultaneously producing the output signal. Advantageously,
recording means for recording said quantity of the output signal
are provided. Also, it is advantageous to provide integrating means
for integrating a signal being proportional to said quantity of
said output signal.
The invention will now be described in more detail while referring
to the accompanying drawings, in which
FIG. 1 schematically shows the basic principal of the
invention,
FIG. 2 exemplifies the dependence of the mass absorption
coefficient .mu. on the photon energy E of the absorbed radiation
for different substances,
FIG. 3 shows a block diagram of a preferred embodiment of an
apparatus according to the invention,
FIGS. 4A, B, C and 5A, B, C show registrations of measuring results
obtained by an apparatus constructed in accordance with FIG. 3,
and
FIG. 6 exemplifies how scanning can be accomplished.
Referring to FIG. 1, a sample 1 is examined, the sample being shown
in cross-section and including a substance b, for instance bone
tissue, and a substance s, for instance soft tissue, by means of a
radioactive radiation source 2 and detector means including a
photon detector 3, two logarithmic intensity meters or ratemeters 4
and 5, two amplifiers 6 and 7, and a subtractor 8. The radiation
source 2 emits photons of two different energies E.sub.1 and
E.sub.2, the mass absorption coefficients of the two substances
being .mu..sub.1b and .mu..sub.1s at photon energy E.sub.1, and
.mu..sub.2b and .mu..sub.2s at photon energy E.sub.2, as shown in
FIG. 2.
The logarithmic intensity meter 4 is arranged to produce a signal
proportional to the logarithm of the incident radiation intensity
I.sub.1 at energy E.sub.1 at photon detector 3 and the logarithmic
intensity meter 5 is arranged to produce a signal proportional to
the logarithm of the simultaneously incident radiation intensity
I.sub.2 at energy E.sub.2 at detector 3. Signal amplifiers 6 and 7
have amplification factors F.sub.1 and F.sub.2, respectively.
Radiation source 2 and photon detector 3 are displaceable relative
to sample 1 in the y-marked direction, whereby the radiation
propagating in the x-marked direction can be swept or scanned
across the sample.
If the radiation intensities at energies E.sub.1 and E.sub.2 when
passing free through the air (indicated by broken lines in FIG. 1)
are denoted I.sub.01 and I.sub.02, respectively, the following
conditions as to the attenuation in the sample will be valid with
some approximation, as is well known:
and
.sub..rho. s and .sub..rho. b being the densities of the
s-substance and the b-substance, respectively, x.sub.s and x.sub.b
being the distances of passage of the radiation through s-substance
and b-substance, respectively, K being a constant, and .mu..sub.1s,
.mu..sub.1b, .mu..sub.2s and .mu..sub.2b being the mass absorption
coefficients mentioned earlier.
Thus, the output signal U.sub.out of subtractor 8 will be
##EQU1##
Provided that ##EQU2## the output signal will be ##EQU3## that is
the magnitude of the output signal will be independent of x.sub.s.
In other words, the detector means will be "blind" as to the
s-substance and produce an output signal which apart from
fluctuations due to the random photon detection only varies
responsive to the content of b-substance.
For the purpose of simplification, suitably, a signal corresponding
to ##EQU4## is subtracted from the output signal according to (4),
whereby the output signal will be ##EQU5##
The above-mentioned simplification, which can easily be obtained by
subtracting a reference generator signal from said output signal
according to (4) or by subtracting or adding said signal in a
suitable way after each of or both of amplifiers 6 and 7, means
that the output signal according to (5) is in direct proportional
to the contents of b-substance of the sample.
The above-mentioned simplified equation (5) as to the output signal
is obtained if each of intensity meters 4 and 5 is arranged to
produce a signal proportional to the logarithm of the ratio between
the intensity of radiation when passing free through the air and
the intensity of radiation when passing through the sample, that
is, proportional to 1n (I.sub.0 /I), and naturally if the radiation
intensifies I.sub.01 and I.sub.02 are chosen such that ##EQU6##
If the radiation is scanned across the sample in the way mentioned
above and the output signal according to (5) is integrated
simultaneously, the following equation as to the resulting signal
is obtained: ##EQU7##
If the scanning speed dy/dt is constant and equals v and the
effective area of the b-substance in the scanning plane is A we
obtain ##EQU8## and ##EQU9## In other words the content of
b-substance in the scanning plane expressed as grams per length
unit (perpendicular to the scanning plane) is proportional to,
signal obtained after integration. The factor of proportionality
can be determined by calculating K.sub.2 and the .mu.-coefficient
expression, the latter from out of a complete knowledge of the
atomic composition of the two substances involved. However, since
linear conditions can be assumed, the factor of proportionality can
be easily determined or the apparatus normalized by means of a
dummy made from the two substances and having a known content of
b-substance.
The preferred embodiment of the apparatus according to the
invention illustrated in FIG. 3 comprises a radioactive radiation
source 12 and a radiation detector 13, which are arranged on a
displaceable fork 14, cf. also FIG. 6, such that the radiation can
be scanned across a sample placed between the radiation source and
the detector. Radiation source 12 is provided with a collimator
such that photons are emitted in a narrow beam towards the detector
13. The detector produces a pulse for each incident photon, the
amplitude of the pulse being proportional to the energy of the
photon. The pulses from the detector are amplified in a pulse
amplifier 15 and thereafter applied to two window discriminators 16
and 17. Said discriminators are arranged to pass well-defined
pulses corresponding to a first and a second photon energy,
respectively, to a logarithmic pulse frequency meter or ratemeter
18 and 19, respectively. The ratemeters produce a DC voltage
proportional to the logarithm of the frequency of the input pulses.
Each of the ratemeters 18 and 19 is connected to a conventional
operational amplifier 22 and 23, respectively, via an adjustable
reference level generator 20 and 21, respectively. These reference
level generators make it possible to add constant DC voltages to
the DC voltages obtained from ratemeters 18 and 19, such that the
input voltages of amplifiers 22 and 23 are null when the photon
beam from the radiation source passes directly through the air to
the detector.
Amplifiers 22 and 23 are coupled to produce output signals having
different polarities. The amplification of amplifier 22 can be
varied by means of the adjustable feed-back resistor R1, while the
amplification of amplifier 23 is constant. The outputs of
amplifiers 22 and 23 are connected to a corresponding input
resistor R2 and R3, respectively, of an operational amplifier 24
coupled as a summator. The amplification of amplifier 24 can be
adjusted by means of an adjustable resistor R4. The DC voltage on
the output of amplifier 24 will thus be proportional to the
difference between the amplified input voltages of amplifiers 22
and 23.
The output of amplifier 24 can be connected via a switch 25 to a
recording unit including a plotter 26, to a circuit 27 for zero
setting control, and to an integrating circuit including a
voltage-frequency converter 28, a threshold circuit 29, and
and-gate 30, a scaler circuit 31, and a circuit 33 for digital
display of the count of the scaler circuit, said circuit 33 being
connected to the scaler circuit via a latch circuit 32.
Threshold circuit 29 is arranged to produce an output signal during
a scanning operation, which enables and-gate 30 and starts the
integration when the input signal to the threshold circuit, that is
the output signal of amplifier 24, in a predetermined direction
exceeds zero voltage by a predetermined value and which disables
said and-gate and stops the integration when the input signal
returns to zero voltage. The predetermined value is chosen such
that noise on the output of amplifier 24 -- inter alia due to the
statistical fluctuations of the photon beam -- cannot be expected
to start the integration procedure.
Circuit 27, which by means of switch 25 can be connected to the
outputs of reference level generators 20 and 21 to control that
zero voltages are obtained when the radiation from the radiation
source 12 falls directly on detector 13, includes an operational
amplifier 34 and an indicating instrument 35. The time constant and
the amplification of amplifier 34 have been increased in a narrow
band around zero voltage by means of a feed-back circuit 36.
All units and circuits used in the apparatus according to FIG. 3
are well known to those skilled in the art, for this reason a more
detailed description thereof should not be necessary. However, as
to the logarithmic ratemeters, these suitably are of the diode pump
pulse rate to direct current converter type.
When using the apparatus according to FIG. 3, first of all,
reference level generators 20 and 21 are set, while directly
irradiating detector 13, such that the input signals to amplifiers
22 and 23 are zero. The zero setting is controlled by connecting
circuit 27 by means of switch 25 by turn to the outputs of the
reference level generators. Thereafter, a sample of the substance,
the influence of which is to be eliminated, for instance soft
tissue, is inserted in the photon beam from the radiation source in
the form of a dummy (for instance water) or a certain part of the
object to be examined and resistor R1 is set such that the output
voltage of amplifier 24 is zero, switch 25 then being in the
position shown in FIG. 3. Finally the total scale factor of the
apparatus is set by means of the adjustable resistor R4. This is
suitably accomplished by inserting a phantom having a known content
of the substance to be examined, for instance bone tissue, in the
photon beam while adjusting resistor R4 until instrument 35 gives a
correct reading, or alternatively until the integrating circuit
upon scanning across the phantom shows a correct value on unit
33.
When the apparatus thus has been normalized or calibrated, the
object to be examined is inserted between radiation source 12 and
detector 13 and the photon beam is scanned across the object. The
output voltage of amplifier 24 is then zero until the photon beam
starts to pass through the part of the object containing the
substance which is the subject of the examination. When the output
voltage due to the absorption of radiation in the substance has
increased to the level determined by the setting of threshold unit
29, and-gate 30 is enabled. Scaler unit 31 now starts to count,
that is to integrate, the number of pulses transmitted from
voltage-frequency converter 28 via the and-gate, the output
frequency of the converter being proportional to the output voltage
of amplifier 24. This procedure continues until the photon beam no
longer passes through the part of the object which contains the
substance subject to examination, that is, until the output voltage
of amplifier 24 is again zero and threshold unit 29 disables
and-gate 30. The count of scaler unit 31 can now be displayed
digitally on unit 33 via latch circuit 32, the count being a direct
measure of the content of said substance. The function of latch
circuit 32 is to prevent that the count displayed is influenced by
any new immediately following integration.
As is shown in FIG. 3, the output signal from amplifier 24 can be
recorded by a plotter 26 during a scanning procedure, whereby a
visual inpression of the distribution of a substance in a sample
can be obtained. In FIG. 4 and 5, there are shown for the purpose
of exemplification registrations obtained by an apparatus according
to FIG. 3. FIG. 4A shows the output voltage of amplifier 24 when
scanning a soft tissue phantom in the form of a cylindrical plastic
vessel filled with water. FIG. 4B shows the output voltage of a new
scanning procedure after the immersion of a bone tissue phantom in
the form of an alumimium tube in the water in the plastic vessel.
FIG. 4C shows the output voltage of yet another scanning of the
aluminium tube immersed in the water but with one isotope channel
(discriminator, pulse counter and reference level generator)
disabled. FIG. 5A, B, and C show the output voltages from amplifier
24 when scanning human leg, arm and fingers, respectively. The
"negative" parts of the registrations indicate the existance of fat
tissue.
In FIG. 6 is shown an example of an arrangement for scanning the
radioactive radiation across a sample (not shown). A radioactive
radiation source 61 and a detector 62 are arranged on one each of
the legs of a fork 63 suspended in a frame construction 64. Said
frame construction comprises means for displacing fork 63 in
accordance with a programmable scanning pattern. Both radiation
source 61 and detector 62 are axially displaceable in a bracket 66
and 67, respectively, arranged on the corresponding fork leg.
Detector 62 is connected to a pulse amplifier (not shown) via a
cable 8.
When using the arrangement of FIG. 6 the part of a body to be
examined is placed between source 61 and detector 62, after which
the source and the detector are adjusted such that suitable
distances between the source and the part of the body and between
the part of the body and the detector, respectively, are obtained.
Thereafter a programmed scanning operation can be made.
Although the present invention is particularly suitable for bone
density measurements, it is obvious that it can be used in other
connections while giving great advantages. Thus, other medical
applications are conceivable where there is a desire to emphasize a
certain type of tissue. Furthermore, the visual registration
according to the invention means that very small structural changes
are seen in a diagram. In other words an increase of the contrast
is obtained, which is evident from a comparison between FIG. 4B and
FIG. 4C.
* * * * *